Conventional automobile theft sensors are illustrated in FIG. 5 through FIG. 7. The sensor shown in FIG. 5 comprises a metal vibrating plate 2 supported on a base 1 at one end thereof, with a weight 3 attached to the other end of the metal vibrating plate 2. In accordance with shaking of the automobile body, the weight is caused to vertically oscillate along with the metal vibrating plate 2, which is made to come in contact with the fixed point of contact 4, forming a closed circuit. By detecting the closure of this circuit at the point of contact 4 a warning signal may be generated at the time of such contact.
The sensor illustrated in FIG. 6 comprises a metal ball 6 on a patterned base plate 5 having a fine electrical pattern etched thereon. When the automobile body rocks or tilts, the metal ball 6 rolls and generates intermittent signals while coming in contact with the etched electrical pattern. In this case, by detecting the intermittent signals, a warning signal may be generated at the time of theft of the automobile or moving of the automobile by a wrecker.
The prior art sensor illustrated in FIG. 7 has a piezo-electric element 8 adhered to a metal disc 7, which is a vibrating disc, and a weight 9 attached to the center of the piezo-electric element 8. By transforming the mechanical automobile body vibrations into an electrical output from the piezo-electric element 8, it is possible to detect theft, and the like, of the automobile.
Even in view of the prior art devices described above, there still exist problems in the art of automobile theft or tampering detection. For example, the problem of false theft detection (false alarms) has not been solved. Furthermore, various tamper related activities result in distinct automobile body vibration patterns. Prior art detectors are generally responsive to limited automobile body vibration patterns associated with a number of tampering activities, and are therefore not responsive to remaining activities.
The sensor illustrated in FIG. 5 has difficulty in responding to the vibration frequencies of various automobile bodies because its resonance frequency band is narrow and therefore the results of detection using this device vary widely among different automobiles. Consequently, the reliability of this device is very poor.
In the sensor illustrated by FIG. 6, the metal ball falls to one side of the base when the automobile is parked with the body tilted to one side, or when the automobile is parked on a hill. As a result, it becomes difficult for the metal ball to roll. Consequently, rocking or vibration of the automobile body, shock, or the insertion of a key into the keyhole cannot be sufficiently detected in these cases. Therefore, the reliability of this device is also poor.
Further, the sensor illustrated in FIG. 7 is wholly inadequate for theft detection purposes. Generally the piezo-electric element of FIG. 7 has a quiet resonance frequency of about 2 Khz. Thus, while the sensor responds easily to street noise, it is not responsive to rocking of the automobile body. Thus in use, this device is not suitable for detecting rocking and vibration of the automobile body associated with theft or tampering.
Other prior art sensors include those adapted to sense the lifting of the automobile by wreckers, jacks, and the like, with the use of rotary encoders and angle detection sensors. However, two or more of these rotary encoders or similar devices were required in order to detect vertical and lateral movement and shaking of the automobile body. Moreover, when the automobile is parked on an incline, the angle of incline must be recorded in a memory device, such that when a change in the angle of incline is detected, computation of the difference in tilt angle may be accurately computed. Thus requires the use of expensive microprocessors and special programming.